A membrane module in which a plurality of hollow fiber membranes is disposed collectively and circularly and one end or both ends of the hollow fiber membranes is fixed by a fixing member with the one end or both ends of the hollow fiber membranes open to form a water collection portion is mounted and used for the sucking filtration apparatus of the immersion type or the filtration apparatus of the external pressure-type.
The membrane module of this kind is widely used in a water-cleaning field such as cleaning of water of rivers, lakes, and marshes. In recent years, there is a tendency for the membrane module to be used not only in the water-cleaning field, but also in a filed of treating highly polluted water, for example, filtration of secondary and tertiary treatment of sewage, drainage, industrial wastewater, and industrial water.
As one application of highly polluted water treatment, a waste water treatment system that uses a membrane bioreactor process (MBR, hereinafter may be referred to as “membrane separation active sludge method”) by means of a membrane module is becoming widely used. Because the membrane separation active sludge method can be carried out for the high-concentration active sludge, the membrane separation active sludge method is capable of decreasing the volume of an aeration tank and eliminates the need for using a precipitation tank and a sludge concentration tank. Thus the membrane separation active sludge method has an advantage that the installation area of the filtration apparatus can be made smaller than that of the filtration apparatus used in a conventional combined water-purifying tank method. In addition, the membrane separation active sludge method has another advantage that it is capable of purifying water in a higher quality than the conventional method.
Regardless of whether it is used for water purification or for highly polluted water treatment, when a filtration apparatus performs filtration treatment continuously by means of a membrane module, suspended ingredient that is contained in raw water, which is water to-be-treated, becomes deposited on the surfaces of membranes or between the membranes. The deposition of the suspended ingredient causes clogging of the membranes. That is, hollow fiber membranes adhere to each other and integrate with each other through a deposit, and the effective membrane area of the hollow fiber membranes inside the membrane module decreases, which leads to a decrease in a penetration flow rate.
Therefore a cleaning operation of removing the deposit on the membrane surface is essential. As a method carried out in most cleaning operations, deposit is peeled from a membrane surface by air bubbling which imparts vibration to hollow fiber membranes by means of bubbles supplied by introducing air from below the membrane module with the to-be-treated liquid filled in an immersion tank and by utilization of the flow of the to-be-treated liquid generated by supplied air.
For example, in Japanese Patent Application Laid-Open No. 7-24264 (patent document 1), as shown in FIG. 26, in the filtration method of a liquid by continually or intermittently performing air bubbling for the hollow fiber membranes 103 by means of bubbles supplied from the air-diffusing hole 102a of the air-diffusing plate 102 provided below the sheet-shaped flat hollow fiber membrane modules 101, the membrane modules 101 are disposed with the sheet surface disposed vertically and the hollow fiber membranes disposed horizontally to vibrate only the hollow fiber membranes 103 by the air bubbling to generate a swirling flow.
In the filter disclosed in Examined Japanese Patent Publication No. 7-61420(patent document 2), as shown in FIG. 27, a large number of the hollow fiber membrane filtration membranes 110 are arranged inside the outer cylinder 111, and the porous pipes 112 are disposed in the bundle of the hollow fiber membranes. Air bubbling is performed by introducing air from below the porous pipes 112, and bubbles are lifted along the porous pipes 112 to generate an upward flow which vibrates the hollow fiber membranes.
[Patent document 1]: Japanese Patent Application Laid-Open No. 7-24264
[Patent document 2]: Examined Japanese Patent Publication No. 7-61420